This invention relates generally to the art of aligning (registering) one article with another. More specifically, and by way of example, the present invention is directed to a method of registering multiple images along an elongated web with respective multiple transfer die of a hot stamping machine.
In a hot stamping operation, one or more portions of a hot stamping foil are transferred at one time to a substrate. In some applications, each transferred portion carries an optical image of some type that is desired to be placed on the substrate. An example of hot-stamping optically variable devices (OVDs), such as holograms is used, although clearly the problem and solution apply to many other continuous web registration applications.
Hot-stamping presses, such as are manufactured by Promeq Inc. and Bobst, are designed to transfer hot-stamping foil from continuous rolls of plain foil to various sheet substrates. In operation, a fixed die or die array causes one or more layers of the foil to be transferred to the substrate, as the substrate is pressed between the die and a reciprocating platen. These presses are normally used to transfer a section of the foil to the substrate to produce a pattern which is the shape of the die. Thus to transfer text, a die made in the shape of the desired text is used. Registration of the web with the die or die array is not an issue is this process, since there are no images on the web that need to be aligned with the individual die. The image is the die itself.
This type of press is now being used to transfer portions of foil along its length that contains images, such as holograms or other OVDs. An example is shown in U.S. Pat. No. 4,728,377--Gallagher (1988), which is incorporated herein by this reference. OVDs are now routinely produced in continuous web form on a hot stamping foil wherein they are nominally equally spaced on the web, each one having an associated registration mark. Various types of registration systems, notably those from Total Register Inc., are used to drive the web such that its stopping position places the OVD image in line with a transfer die, so that image is transferred from the web to the desired location on the substrate. This substrate, which is most frequently in sheet form, is typically either plastic, as in the case of the familiar credit cards; or paper, as in magazine covers such as National Geographic.
In the interest of clarity, in the discussions that follow, all references are to a single web system. In practice, multiple webs are used in parallel. Each of the multiple webs behaves independent of the others; therefore, the discussions apply equally well to multiple web systems.
Most of this stamping is done in a flat-bed hot-stamping machine using a single die such that one image is transferred for each cycle of the stamping press. In some cases multiple dies on the same web are used, but current registration systems do not provide sufficient accuracy over the long distances required to utilize large numbers of dies. Clearly, if multiple dies can be used, productivity will increase roughly in proportion to the number of dies. Current registration systems provide registration of the web at a single point for each impression. Two such existing arrangements are illustrated in FIGS. 1(a) and 1(b) herein. Therefore, registration accuracy at all other points is subject to variation in registration due to two factors: variation in spacing of the images along the length of the web due to web stretch during the web manufacturing process, and variation in spacing of the images due to web stretch during the hot stamping process.
Attempts to solve this inadequate registration to date have centered on improvements in the accuracy of web tension within the hot stamping press. While this has resulted in improvements, it has not proven to overcome the problem, since web tension is not the only contributor to web stretch. Other factors affecting elasticity play a significant part; notably the gauge (thickness) of the web, and the variation in modulus of elasticity as a function of temperature. This latter effect is surprisingly large. It is quite conceivable that there are sufficient temperature variations within a typical hot stamping press to cause the modulus of elasticity to vary by a factor of two. Thus, for a given web tension, the amount of stretch can vary by a factor of two.
These effects are of no significance in a well designed registration system when only one die is being used. Nor are they significant where a small number of dies are being used, or when image position tolerances are large. They become extremely significant, however, when attempting to hot-stamp OVDs on such things as bank notes, where the large sheet sizes inherently have large distances from the first die to the last, and the images are so small that very slight errors in registration are very obvious to the unaided eye. To some extent it is desirable to make register requirements difficult in order to deter counterfeiting.
Therefore, it is a primary object of the present invention to provide a technique of controlling a web such that registration between the web images and a hot stamping die array is maintained over large distances, despite long-term variations in pitch and elasticity of the web material.
It is a more general object of the present invention to provide a technique for accurately aligning and registering each of multiple locations along the length of an elastic web with a plurality of fixed positions.